Diving, by itself, can be a strenuous activity. When the moving of equipment or other heavy work is to be performed, a diver often labors just short of fatigue. Now that saturation diving and related techniques permit diving operations to be performed at extreme depths, dense, sophisticated breathing mixtures have thrust another load upon the diver. The increased masses and viscosities of these mixtures require considerable energy for inspiration and expiration. While at the surface, the effort exerted for breathing is small and is generally taken for granted, but at great depths this effort is of such a magnitude as to seriously impair a diver's ability to function effectively. It has been shown by research and diving tests, that the work of diving increases with an increase in the density of the diving mixture. The gaseous exchange between the tracheo-bronchial tree alveoli and the vasculature decreases under increased work load due to the respiratory efforts increasing. That is, with the depth of respiration decreasing, an increase in dead space occurs within the trachea and bronchi, and respiratory fatigue sets in due to inadequate O.sub.2 CO.sub.2 exchange occurring. The respiration becomes more rapid but shallower and, therefore, a cycle of decreased gaseous exchange occurs. Muscle fatigue then sets in rapidly and the work load is decreased.
In one example of a simple test that produced fatigue, two divers were placed in a hyperbaric test chamber which was pressurized to a simulated depth of around 600 feet. Although simple tasks were to be performed the divers soon became fatigued. Tightening a nut on an overhead bolt left them gasping and they had to rest twice before the bolt was tightened. The great effort expended to breathe the dense gas mixture, coupled with the work of raising the arms and hands overhead to tighten the nut were incapacitating. Obviously, their job effectiveness was marginal, largely due to the breathing effort.
One attempt to reduce the effort imposed by breathing the dense gas mixture calls for forcing the mixture into the diver's lungs. A suitable sensing device, similar to a demand regulator diaphragm senses the beginning of the normal inhalation of gas. The sensor would then trigger a pump, which would force more of the mixture into the lungs. Usually, this initiates a strangling or gagging response and it is up to the diver to exhale unassisted. Any diver who has used such a system will attest that it is manifestly uncomforable and psychologically disturbing. From the physiological standpoint, experiments have shown that there is an interference with the intrapulmonary pressure within the pleural cavity. The interference is actually in the form of an increase in the intrapulmonary pressure which directly depresses the blood flow. It has been observed that when using this forced gas technique and applying a suitable ventilatory pressure to double the tidal volume of the gas mixture in the lungs, the blood flow was depressed as much as 14 percent. Thus, the cardiac output is reduced, the pulmonary artery resistance increased, and the dynamics of the chest are altered significantly.
To reemphasize, the forced gas method or iatrogenic type of respiration is abnormal for it is directly opposite to the normal respiratory rhythm. The breathing mixture is forced to the tracheo-bronchial tree under positive pressure to insufflate the lungs, aveolar sacs, etc. The diaphragm is pushed downward by the forced gas rather than the diaphragm's drawing in the breathing mixture. A consequence of this pressing of the air into the respiratory tract under pressure is that intrapulmonary pressure within the cavity is increased. The heart is partially collapsed, the cardiac output is depressed and the culminary artery resistance is increased. As a result, blood flow is reduced. This result is unacceptable particularly for a diver who must work to his limits. There is a continuing need in the state-of-the-art for a diver aid which eases the burden imposed by dense breathing gas mixtures and yet does not adversely psychologically or physiologically affect its user.